Is an item inside a bag of holding still considered to be in the same plane?

Question

Multiple features and items have text specifying they only work if the "target" is located on the same plane, so if said target (object or creature) entered a bag of holding, would an ability that specifies "same plane" still work?

Answer 1

No is is not

This has been explicitly answered by the Sage Advice Compendium:

Are extradimensional spaces, such as a demiplane or the space created by rope trick, considered to be on a different plane of existence? An extradimensional space (aka an extraplanar space) is outside other planes. Therefore, if you’re on the Material Plane and your foe is in an extradimensional space, the two of you aren’t on the same plane of existence.